They have saddles, and something called a "crop bra/chest protector" which might be what you want.
There is also a thread here on BYC with a free pattern for the saddle. search for Free chicken apron/saddle pattern. They aren't really hard to make, and you could customize it for your chicken. And they don't care how it looks!
I make mine out of sunbrella fabric.
I have only ever used plain saddles and saddles with the attachment to protect their shoulders.
I had to put a piece of elastic around the neck of one of my saddles as I had an escape artist hen. The elastic was loose, but tight enough to hide in her feathers so she wouldn't peck at it or get it stuck in her beak. It did not restrict her neck or throat at all.
As long as it isn't TIGHT I would think it's ok. Snug enough to stay where you want it, but not restrict anything.
You certainly don't want to strangle her or impact her digestion. Perhaps contact the maker of the ones above and see how they actually fit?
I make my own saddles, and adjust back length and elastic lengths based on the breed and size of chicken. Once you know how they go together, it's pretty easy to customize them. And looks are not real important, just that it gets the job done. If you can't sew a perfect seam, no one in the coop will care! Hope that helps.
I'm glad they were helpful! Hope you are able to resolve her troubles. When mine wear saddles, it usually takes quite a while for the feathers to fill in enough to remove them again. I made my issue better by increasing my hen to rooster ratio. They tend to get a little less attention that way, and hold up better!